Fuel regulating valve with fluidpressure servo responsive to ratio of two pressures



Aug. 24, 1954 N. E. STARKEY 2,687,273

FUEL REGULATING VALVE WITH FLUID-PRESSURE SERVO RESPONSIVE TO RATIO OF TWO PRESSURES Filed June 15, 1951 2 Sheets-Sheet 1 .s'imW/va moms IT'1VQTI'COT": Neal ELStaTfKe y.

His AtoPne5 Aug. 24, 1954 N. E. STARKEY 2,537,273

FUEL REGULATING VALVE WITH FLUID-PRESSURE SERVO RESPONSIVE T0 RATIO OF TWO PRESSURES Flled June 15 1951 2 Sheets-Sheet 2 m M l'l W M 3 IN 10 I l 4 I l Inve ntor: Neal E.St!aT-Ke g,

His Aet'oTne H.

Patented Aug. 24, 1954 FUEL REGULATING VALVE WITH FLUID- PRESSURE SERVO RESPONSIVE TO RATIO OF TWO PRESSURES Neal E. Starkey, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York Application June 15, 1951, Serial No. 231,703 (01. 25128) 4 Claims.

1 This invention relates to fluid fuel regulating systems, particularly to a fluid-pressure servomotor-operated throttling valve arranged to maintain a preselected ratio between two pres- 2. pressure-restored follow-up mechanism for stabilizing the action of the fluid-pressure servomotor which positions the pressure ratio controlling valve.

sures occurring in a fluid pressure system such as 5 Other objects and advantages will become apthe fuel supply system of a gas turbine driven parent from the following description taken in powerplant. connection with the accompanying drawing, in

The internal combustion turbine powerplant is which Fig. 1 is a diagrammatic representation of found well suited to use natural gas or other a gas turbine powerplant using fuel gas and havcombustible gas as a fuel. The required gas presing a pressure ratio regulating valve incorporatsure varies from 1 to 150 pounds per square inch, gage, depending on the rate of air flow in the combustion chambers. As will be appreciated by those skilled in the art, the gas turbine is provided with a metering valve for regulating the flow of fuel to the combustion system at rates dictated by complex servo-mechanisms having components responsive to rotor speed, the temperature level at which the plant is operating, the load output desired, and various other pressure and/or temperature conditions obtaining within the powerplant. In order to simplify the control system for the fuel regulating valve, it is desirable to control the inlet pressure to the fuel valve so as to be proportional to the air flow in the combustion system. The pressure in the combustion system is proportional to the combustion air flow, consequently the inlet pressure to the fuel valve should preferably be made proportional to the combustion space pressure to obtain the desired proportionality to air flow. In a gas ing the invention, Fig. 2 is a full sectional view turbine powerplant for gas pipe-line pumping,

for instance, the fuel gas is obtained from a source the pressure of which varies from 150 to 250 pounds per square inch, gage. Consequently, there is a need for introducing means for reducing the supply line pressure to values more suited to the requirements of the turbine.

Accordingly, the purpose of the present invention is to provide an auxiliary fuel gas pressurereducing valve arranged to maintain a constant pressure ratio across the gas turbine regulating valve. More specifically, the special throttling valve which comprises the essence of this invenof the pressure ratio control valve itself, and Fig. 3 is a detail view of the pressure ratio adjusting means.

Generally stated, the invention is practiced by providing a specially arranged hydraulic or other fluid-pressure actuated servo mechanism for positioning a pressure reducing valve, the servo mechanism having pressure responsive components sensitive to the discharge pressure of the valve and to the combustion space pressure of the powerplant, these pressure responsive components acting in opposition on a pilot valve.

Referring now more particularly to Fig. 1, the pressure ratio control valve is shown applied to a gas turbine powerplant of the simple opencycle type, comprising an axial flow compressor indicated generally at I, a combustion system comprising a plurality of cylindrical combustors spaced circumferentially around the axis of the powerplant and indicated generally at 2, and an axial flow turbine, the exterior casing of which is indicated at 3. The compressor takes air from the surrounding atmosphere through an intake casing la and discharges directly into the combustors, from which hot motive fluid enters the turbine 3 and leaves through an exhaust casing 3a. The structural details of this gas turbine powerplant are not material to an understanding of the present invention; but it may be noted that the powerplant shown is as disclosed in the copending application of Alan Howard, Chester S. Rice, Bruce 0. Buckland, Serial No. 754,002, filed June 11, 1947, now Patent No. 2,650,753,

tion is adapted to maintain at. a constant preissued' September 1, 1953, and assigned to the selected value not the difference but the geometric same assignee as the present application. This ratio between the pressure of the fuel gas suppowerplant is a self-contained assembly, capable plied to the powerplant regulating valve relative of being supported at its mid-portion by a pair of to the combustion space pressure. This very transversely spaced, vertically extending links 4 greatly simplifies the design problems encounpivoted at their lower ends to the foundation of tered in providing a suitable fuel valve with the proper servo mechanism to control it in accordance with the requirements of the gas turbine.

the machine, with a third point of support being furnished by a fixed pedestal 5. This support arrangement for the powerplant is disclosed more completely in the United States Patent of Alan Howard and Bruce 0. Buckland, No. 2,632,997,

issued March 31, 1953, and assigned to the same assignee as the present application.

The powerplant auxiliaries comprise a suitable starting motor indicated in Fig. 1 as being an electric motor 6 coupled to the exhaust end of the turbine-compressor rotor. This same end of the rotor may be provided with a coupling '1 for delivering the useful power output of the plant to the gas turbine pipeline compressor or other suitable load.

At the inlet end of the compressor are grouped a number of auxiliaries including a regulator 8 adapted to provide a variable control signal pressure for determining the rate of fuel supply to the combustion system. The details of this regulator 8 are not material to an understanding of the present invention, but it is a hydraulic-mechanical servo mechanism which may be responsive to numerous operating characteristics of the powerplant, such as rotor speed, motive fluid temperature, ambient atmospheric temperature and pressure, and the position of a control member which is set, either automatically or manually, to select the power output required. This lastmentioned control member is indicated diagrammatically in Fig. l as being a manual throttle lever 8a. Regulators of this general type are dis closed in the United States patents of Edwards, Garr, and Ogle, No. 2,622,393, issued December 23, 1952, and Starkey, Lewis, and Edwards, No. 2,558,592, issued June 26, 1951, both assigned to the same assignee as the present application. These regulators comprise means for integrating a number of input signals and producing in accordance therewith a hydraulic output signal pressure which is a function of the rate of fuel supply desired. This signal pressure is represented by the symbol VCO (variable control oil pressure) communicated through the pressure sensing conduit 9.

Fuel gas at pressure P1 is taken through a supply conduit lll from the main gas pipeline (not shown) through the special pressure ratio controlling valve indicated generally at H, through conduit portion Illa to the main metering or throttle valve 12, and through conduit portion lflb to a fuel manifold 10c, thence by way of branch conduits ltd to a plurality of fuel nozzles l3, one of which is associated with each combustor.

As will be apparent from Fig. l, the fuel regulating valve 12 incorporates a fluid pressure servomotor, indicated diagrammatically as comprising a flexible bellows 62a, to which the VCO signal pressure is communiiated by conduit 9.

The flow control disc l2b of the fuel valve is accordingly positioned by the signal pressure VCO.

In accordance with the invention, the auxiliary pressure ratio valve I! is arranged to maintain a constant geometric ratio between the discharge pressure P2 in conduit Illa and the pressure Pc existing in the combustion chambers 2. This combustion space pressure is substantially equal to the discharge pressure of compressor l, and is communicated to the valve i I by a pressure sensing conduit Hi.

The hydraulic servo mechanism which operates the pressure ratio valve H may be seen in Fig. 2. The main valve housin i5 defines an inlet chamber [5 having an inlet port lfle and an outlet chamber H with a discharge port 18f. Disposed between the inlet and outlet chambers is a valve seat member is forming a Venturishaped passage and retained in place by a snap- 5 ring R9 or equivalent fastening means. The fluid 7 throttling member comprises a valve disc assembly indicated generally at 28 as comprising an outer head member 2! with a contoured outer surface adapted to cooperate with the venturi l8 to form a streamlined annular throttling orifice. A central opening 22 in the member 2i is adapted to be closed by an inner head member 23 secured to a valve positioning rod 24 As shown in Fig. 2, the end head member 23 is threadedly received on the end of rod 2 1 and secured by a lock-nut 25. It will be apparent to those skilled in the valve art that this valve disc assembly constitutes a well-known type of flow control member used where the operating pressures are suiilciently high as to require a force of comparatively large magnitude to open the valve against the full inlet pressure. Initial upward movement of rod 24 causes the inner head member 23 to open the port 22 so. that a limited amount of fluid can flow through the central opening 22 in the main flow control member 2i. This limited iiow permits some equalization of the pressure drop across the valve so that less force is required to move the outer head member 22. Upward movement of the rod 2% eventually causes the inner head member 23 to engage the threaded plug member 25, after which further upward movement of rod 2t causes the main flow control member 21 to rise. Such high pressure valve arrangements are well known to those skilled in the and need not be further described herein.

The small auxiliary valve disc 23 also serves to control the iiow at low flow rates with high differential pressure. This provides a wider range of flow control than can be obtained with a single large valve due to the manufacturing tolerances which are necessary.

The hydraulic servomotor for positioning the valve disc assembly 26 comprises a hydraulic cylinder indicated generally at 2'6, controlled by a servo pilot indicated generally at 28.

The hydraulic motor 27 comprises a housing portion 15a defining a cylindrical bore 21a in which is disposed a stationary bushing 29. This bushing assembly may conveniently be retained in position in bore 23a by means of a pair of retaining snap-rings 36, 3| engaging opposite end portions of the bushing. Bushing assembly 29 has a lower reduced end portion 32 formed separately so that the piston can be installed, and defining a recess housing a metal packing and scraper ring member 33. This member 33 is provided with a central bore having annular grooves forming teeth 34 which form close clearances with the rod 24 and serve to clean the axially moving rod of any dust or dirt particles which accumulate thereon from the in-flowing fuel gas. Thus, this dirt is prevented from traveling along the rod 24 and getting into the hydraulic actuating fluid.

The central portion of bushing 29 forms a recess 35 in which is slidably disposed the output piston 36. This is of course fastened to or formed integral with the piston rod 24 and may be provided with a circumferential groove containing a resilient packing ring 31. At its upper side, piston 36 is provided with an axially extending portion 38 which slidably projects through a bore portion 35a and has a still further reduced diameter end portion 39 which projects upwardly into a, sight glass indicator 40, arranged so that the position of the piston rod extension 38 will indicate the degree of opening of the valve 20. The sight glass may of course be suitably graduated to read inches of travel of the flow control membe threadedly received in a removable access opening cover plate 4|, secured by threaded fastenings 42 to the housing wall portion defining a generously proportioned access opening 43, the function of which will be seen hereinafter.

Disposed in the bore 35 surrounding the piston rod extension 38 is a main biasing spring 44, which strongly urges the flow control valve to the fully closed position. Hydraulic fluid for positioning piston 36 in opposition to spring 44 is admitted to the bore by way of a pair of supply and drain passages 45, 46. The upper passage 45 communicates by way of an annular groove 41 in the outer surface of bushing 29 and one or more radial holes 48 with the bore portion 35, so as to act on the upper surface of piston 36.

The lower passage 46 communicates by way of an annular groove 49 and one or more radial grooves 50 with the lower surface of piston 35.

The pilot valve assembly 28 serves to control the fiow of hydraulic operating liquid to and from the motor 2'! by way of the passages 45, 45 in the following manner. The pilot spindle 5! is provided with a plurality of axially spaced lands 52, 53, 54 adapted to cooperate with axially spaced ports in a stationary bushing 55. Land 52 is normally adapted to cover ports 56, communicating with an annular groove 51 and a drain passage 58. The intermediate land 53 normally covers a port 59 communicating with an annular groove 60 to which is supplied hydraulic operating liquid at a suitable pressure by way of an inlet conduit 6|. This hydraulic liquid is supplied at a pressure on the order of 300 pounds per square inch from a suitable hydraulic pump, which is not shown but may conveniently be driven from the turbine rotor as one of the auxiliary devices associated with the regulator 8. The upper pilot spindle land 54 ordinarily covers drain ports 52 which communicate with an annular groove 53 and a drain port 64. Bushing 55 is also provided with lower ports 55 and upper ports 65.

It will be apparent from Fig. 2 that the pilot spindle 5i is adapted to slide vertically in bushing 55, which is held stationary in bore 55a by a lower snap-ring retainer 6! and an upper snapring 68. i

In order to insure that dirt will not enter the hydraulic circuits, and to prevent contamination of the hydraulic actuating liquid by the fuel gas flowing through the valve, high pressure control oil from inlet conduit 5| is supplied from the groove by way of a passage shown partly in dotted lines at 65 to an annular groove 18 in the outer surface of bushing 28, thence by way of one or more radial passages H to an annular recess 12, sealed by two O-ring packi ngs 12a and 12b, located intermediate the piston rod. cleaner member 33 and the piston 36. With this arrangement, high pressure oil in groove 12 tends to lubricate the two rings 12a, 12b, and if, for any reason, these packings leak, the high pressure oil will leak into the gas, rather than gas leaking into the lubricating system of the turbine.

The pilot spindle 5! is adapted to be positioned by a pair of opposed pressure responsive bellows assemblies indicated generally at 13 and 74 respectively. The lower bellows assembly 13 comprises a housing 13a which may conveniently be fabricated separately from the housing It and secured thereto by threaded fastenings 75 with a bushing member 16 clamped therebetween.

6 Housing 13a defines a cylindrical recess 11 into the open upper end of which bushing 16 projects. Disposed in recess 11 is a. pressure responsive piston member 18 biased upwardly by a spring 19 and sealed to bushing 16 by a flexible bellows 80. Combustion space pressure P0 is communicated by way of conduit l4 to a port 7312 so that piston V 18 is exposed to the combustion chamber pressure. The upper end portion of piston 78 is provided with a,ball-point abutment member 8|, which may be threadedly received in the end of piston 18 so as to be capable of adjustment. This ball-point member engages freely the end surface of the pilot valve land 52. The second pressure-responsive bellows assembly 14 comprises a housing member Ma, defining a cylindrical recess 82 in which is disposed a piston member 83 biased downwardly by a coil spring 84, and sealed by a bellows member 85 to a bushing member 8 6. Bushing 86 is secured by threaded fastenings 31 between the housing Ma and the upper housing portion 51), the latter being a cap member secured by a plurality of threaded fastenings 88 to the main housing member 85a.

As will be apparent from Fig. 2, the respective ends of bellows 85 are sealed to the end head portion of piston 83 and to the upwardly projecting portion of bushing 85. The lower end portion of piston 83 project through bushing 86 and carries a ball-point abutment d9 which may also be threadedly received in the end of piston 83 so as to be adjustable. As indicated in Fig. 1, the exit pressure P2 in the conduit Illa is communicated to the pressure responsive assembly 14 by way of a pressure sensing conduit leg.

The upper pressure responsive piston 33 does not bear directly on the pilot spindle 5i but acts through the agency of a variable ratio arrangement comprising a pivoted lever 96 supported at its right-hand end on a normally fixed fulcrum 9|. The ball-point abutment 89 bears against the upper surface of lever 98, and the lower surface of the lever rests on the spherical end of a piston 92, which slides in the upper portion of bore 55a. Piston s2 engages the upper end of pilot spindle 5| by a push-rod 93 having an upper spherical end engaging the inner surface of piston 92 and a lower reduced end portion having a conical or spherical end disposed in a recess in the upper pilot land 54.

Thus, it will be apparent that the pilot spindle 5l' is positioned by a force proportional to the combustion space pressure PC acting in opposition to the fuel supply pressure P2. It is to be particularly noted that the upper ball-point abutment 89 engages the lever 5E0 at a location closer to the fulcrum 9| than the point of engagement between lever 99 and piston 92. Because of this inequality of the lever arms, the fuel supply pressure P2 must be greater than the balancing force provided by the combustion space pressure Po. This ratio may be adjusted somewhat by moving the fixed fulcrum 9| toward or away from the point of engagement with the ball-point 89 and piston 92. The means for effecting this adjustment is as follows.

The fulcrum 9| consists of a pivot carried in a yoke member 94, a top view of which is shown in Fig. 3. This yoke is formed integral with a slide member 95 having longitudinal edge portions 95a in dovetail engagement with a pair of transversely spaced guides 95. Slide t5 defines a rectangular opening 9! in which is disposed an eccentrically mounted cam 58 adapted to rotate on an upward extension 99 of the stationary bushing member 29. The cam may be retained on this extension 99 by means of a snap ring I60 (see Fig. 2) or equivalent retaining means. Two holes l9! are provided in cam 98 so that a suitable tool may be applied to the cam in order to adjust it rotatably about the projection 99, which action causes the slide 95 to move longitudinally in the guides 95 so as to move the fulcrum iii to or from the ball point 89. When the desired adjustment is obtained, a locking-screw 162 is tightened.

It will be apparent that moving the fulcrum 9| toward or away from piston 92 and ball-point 89 will change the relative length of the lever arms on which these members operate. Thus, by changing the mechanical advantage by which the piston 83 acts on the pilot 5!, the geometric ratio maintained between the fuel supply pressure P2 and the combustion space pressure PC may be altered, for instance, from a value of about 1.2 to a value of about 1.5 with the mechanical arrangement shown in the drawings. It will also be observed that this adjustment is made by removing access plate I, loosening lockscrew I82, and adjusting cam 98. This adjustment may easily be made, even while the powerplant is operating.

In order to stabilize the operation of the hydraulic servo mechanism, a pressure responsive follow-up mechanism is provided, as follows. The upper pilot drain chamber comprises a cylindrical recess 103 in which is disposed a follow-up piston I84, carried on a rod N5, the upper end of which carries a threaded fitting Hi5 with a transverse pivot Hl'l forming a pin and slot connection with the end of lever 56. Piston rod 195 passes through a bushing member I08 retained by snap-ring or equivalent retaining member. It will be apparent from Fig. 2 that the upper surface of follow-up piston Hi l is exposed to the pressure of hydraulic operating liquid in the drain chamber N13. The lower surface of piston 104 is exposed to the pressure of liquid in the drain chamber 58 by reason of a communicating passageway 5811.

It need now be observed that spent operating liquid is drained from the lower chamber 58 by a restricted orifice 58b, the effective area of which may be varied by adjusting a conical pointed needle member I09 formed as a reduced end portion of a threaded adjusting screw 5H]. It will be apparent that the effective area of orifice 581) can be readily adjusted by turning set screw Hi] and looking it with nut Hi.- Similarly, spent liquid is drained from the upper chamber I03 by way of a restricted port [03a with which cooperates an adjustable needle I 12 carried on a threaded adjusting screw H3 having a lock-nut Ht. Liquid discharged from ports; 5%, 103a leaves by way of a communicating passageway H55 and drain conduit H6.

The operation of this hydraulic servo mechanism is as follows. Assuming first that the power plant is at rest with the flow control valve member 21 in closed position, under the influence of hydraulic motor biasing spring 44 and the pressure of the fuel gas supplied to inlet port Hie. Since the powerplant is not operating, there will be no hydraulic control oil pressure supplied to the pressure ratio valve H through conduit 6|, nor will there by any VCO regulator oil pressure to act on control valve i2, which is therefore also closed. Thus, the pressure ratio valve H and fuel regulating valve l2 provide dual protection against fuel gas entering the powerplant and forming an explosive mixture therein when the plant is shut down. When it is desired to start the powerplant, electric motor 6 is energized and the compressorturbine rotor caused to turn, with the result that the hydraulic pump associated with the fuel regulator 8 supplies control oil under pressure to conduit ti. It need now be noted that the coil springs 19, as associated with pressure sensing pistons i8, 83 respectively are so selected, and the adjustable ball-point abutments iii, 89 are so adjusted that, in the shutdown condition, the pilot spindle 5i will be slightly elevated so control oil from conduit 6| flows through port 59, past the intermediate land 53, by way of port 55, and passages 45, d9, 50 to the under-surface of piston 36, causing it to start upward. The resultant flow of fuel gas causes the pressure P2 to rise. This produces a downward biasing force on piston 83, which tends to restore pilot 5! so as to cut off the supply of operating liquid to the underside of piston 36.

Meanwhile, the stabilizing eiiect of the followup piston His is produced as follows. As piston 36 moves upwardly, the hydraulic liquid occupying the space above the piston is forced out through passage d5, port 66, and past the upper land 5d through port 52 and groove 63 to the drain port 6%. Now it will be seen that the rate at which this spent liquid is permitted to escape from drain chamber 103 is controlled by th adjustable orifice Hide. Because'of this restricted discharge from the drain chamber 103, a pressure will be maintained on the upper surface of the follow-up piston Hi4. This introduces an additional downward biasing force on lever 9t, which force is a function of the velocity of upward movement of the output piston Thus, the follow-up piston Hi l helps restore the pilot spindle 5!. to its steady-state neutral position somewhat before the fuel supply pressure P2 is adequate to center the pilot.

Conversely, when the powerplant is shut down, or when the pressure ratio Pz/Pt becomes too high for any other reason, the pilot 5i moves downwardly, so that operating liquid is supplied from conduit 6i by way of port and passage 45 to the upper surface of piston it, The resulting downward movement of piston 35 expels liquid through grooves Ed, ts, passage 46, port 65, and past the lower land 52 to port 56, groove 5?, and discharge chamber 58. Now the restricted orifice 5817 causes a pressure to be built up in chamber 58 so that a force is exerted upwardly on the bottom surface of the follow-up piston 554. This introduces an upward biasing effect on lever 953 which helps restore the pilot SE to its aligned condition.

Thus, it will be apparent that the follow-up piston E84 introduces a stabilizing force proportional to the rate of movement of the flow control Valve 29. It will also be apparent that this stabilizing efiect may be made different for either direction of movement of the output piston 36, by separately adjusting the set screws H5), H3, so as to provide different degrees of restriction of the orifices 5%, E la.

It may further be noted that the biasing springs i9, 84 and the adjustable ball-point abutments 8!, as are preferably selected and adjusted so that the pilot will normally be positioned to maintain the flow control disc assembly 2t partially open, so that the supply pressure P2 will never fall below some preselected minimum value, for instance on the order of 4 or 5 pounds per square inch gage, that being the minimum pressure desired in starting the gas-turbine powerplant.

Thus, the invention provides an effective fluidpressure servo actuated valve for maintaining constant at a preselected value the relation between the fuel gas pressure supplied to the gas turbine regulating valve, relative to the combustion chamber pressure.

It will be obvious to those skilled in the art that many changes and substitutions of equivalents may be made in the mechanical details of this pressure ratio control valve; and it is of course desired to cover by the appended claims all such modifications as fall within the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent in the United States is:

1. In a servomotor the combination of fluidpressure motor means with a reciproeable pilot valve spindle member having fiow control portions adapted to control the motor, first expansible chamber means adapted to exert a force on said pilot proportional to a first signal pressure, second expansible chamber means adapted to exert a second force on the pilot spindle in opposition to said first force and proportional to a second signal pressure, and variable ratio lever means interposed between one of the pressure responsive means and the pilot spindle, said variable means comprising a lever disposed generally normal to the pilot spindle and having one side portion engaged by said last-mentioned pressure responsive means and an opposite side portion engaging the pilot spindle, normally fixed fulcrum means supporting said lever at one end thereof, said points of engagement of the opposite sides of the lever being at different distances from said fulcrum, and means for adjusting the fulcrum toward and away from said points of engagement whereby the mechanical advantage with which the respective pressure responsive means acts on the pilot spindle may be varied.

2. Servomechanism in accordance with claim 1 and including pressure-restoring follow-up means for imposing a restoring force on the pilot spindle as a function of the rate of movement of the fluid-pressure motor, said followup means comprising means for restricting fluid flow through the respective drain conduits from the pilot spindle for the corresponding directions of movement of the fluid-pressure motor, a follow-up piston connected to exert a restoring force on the variable ratio lever, and conduit means for subjecting opposite sides of said follow-up piston to the pressure of the fluid in said respective drain conduits at the upstream sides of said flow restrictions.

3. A pressure ratio controlling valve comprising throttling means for reducing the pressure in a fluid conduit, hydraulic motor means for positioning the throttling means, a pilot valve spindle member adapted to control the hydraulic motor, first pressure responsive means adapted to exert a force on said pilot proportional to a first signal pressure, second pressure responsive means adapted to exert a second force on the pilot spindle in opposition to said first force and proportional to a second signal pressure,

variable ratio lever means interposed between one of the pressure responsive means and the hydraulic pilot spindle, said variable ratio lever having one side portion adapted to be engaged by the pressure responsive means and an opposite side portion adapted to engage the hydraulic pilot spindle, normally fixed fulcrum means supporting said lever at one end thereof, said points of engagement with opposite sides of the lever being at diiferent distances from said fulcrum, and means for adjusting the fulcrum to and from said points of engagement whereby the mechanical advantage with which the pressure responsive means acts on the pilot spindle may be varied, and pressure restoring follow-up means for imposing a restoring force .on the pilot spindle as a function of the rate of movement of the hydraulic motor, said follow-up means comprising means for restricting fluid flow through the drain conduits from the pilot spindle for the respective directions of movement of the hydraulic motor, a follow-up piston connected to said variable ratio lever, and conduit means for subjecting opposite sides of said follow-up piston to the pressure of the fluid in said respective restricted drain conduits.

4. In a servomotor for positioning a movable member in accordance with the geometric ratio between two fluid pressure signals, the combination of motor means having a pilot member adapted to control the motor, first expansible chamber means adapted to exert a force on said pilot proportional to a first signal pressure, second expansible chamber means adapted to exert a second force on the pilot in opposition to said first force and proportional to a second signal pressure, and ratio-determining lever means interposed between at least one of the pressureresponsive mean and the pilot and comprising a lever member disposed between the pressureresponsive means and the pilot and having one portion engaged by said last-mentioned pressure-responsive means and another portion engaging the pilot, and fulcrum means pivotally supporting said lever member, said points of engagement with the lever being spaced at different distances from said fulcrum means.

References Cited in the file of this patent UNITED STATES PATENTS Number 7 Name Date 1,052,588 Janioki Feb. 11, 1913 1,904,475 Kissing Apr. 18, 1933 2,237,070 Cleveland Apr. 1, 1941 2,422,808 Stokes June 24, 1947 2,438,663 Greenland Mar. 30, 1948 2,451,029 Hughes Oct. 12, 1948 2,508,260 Holley May 16, 1950 2,515,074 Bobier July 11, 1950 2,531,780 Mock NOV. 28, 1950 2,579,334 Plank Dec. 18, 1951 2,581,275 Mock Jan. 1, 1952 2,590,853 Fulton Apr. 1, 1952 2,604,756 Greenland July 29, 1952 2,616,254 Mock NOV. 4, 1952 FOREIGN PATENTS Number Country Date 634,095 Great Britain Mar. 15, 1950 

